Hi Evaldas! Welcome to permies! Your english is fine!
I did a little search, and I think the little worm might be fruit fly larvae.
I think it might be a little cool to take some soil and the worm and watch it grow up in a little glass jar, as a science experiment to watch its life cycle.
I think it might be too early in the mandarin tree's life to know what may or may not be a problem. I think planting the mandarin tree with lots of flowering plants might help to mitigate any potential pest problems.
are there any white things on the tree itself, or only on the soil?
particularly, did it fall off your tree?
I agree, put it in a jar and see what it becomes.
As for flowering plants, not sure what Dave has in mind but I have oranges and lemons planted in large pots together with marigolds (tagetes) in the same pot. it doesn't keep everythign away, but it keeps the dirt surface covered (less stress on the orange) and the chewing insects don't like the smell too much.
I think some flowering plants that could work for repelling pests and attracting beneficial insects include:
If it is possible to make long-term installations on your property, I think planting the mandarin tree outside and the flowers in a circle around it would be awesome, because then, fungal and root associations could start to develop, as well. The plants would be able to talk with each other.
If it is not possible to make more permanent installations, I think whatever you can do will work! If they are in separate pots, I think the benefits of attracting beneficial insects and repelling pests will still happen, because I think those parts are more scent based . It's like the plants send out pheromones of some kind. I might be concerned about the plants crowding each other if they're sharing the same pots, so, if there is something you like to cook with that is highly aromatic, like basil, thyme, cilantro, parsley, or garlic, then, I think it could work having them in the same pots. Because, I think those plants grow pretty quick, so being around to harvest a little of them every couple to use in your cooking ought to keep the plants in check.